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Cynoscion regalis Etrumeus teres P eprilus triacanthus. Sphoeroides maculatusUID and other fi shesRock crabs. Ovalipes ocellatus UID and other crabs Mysids.

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Abstract— Piscivorous fishes, many of which are economically valuable, play an important role in marine ecosystems and have the potential to affect fish and invertebrate populations at lower trophic levels. Therefore, a quantitative understanding of the foraging ecology of piscivores is needed for ecosystem-based fishery management plans to be successful. Abundance and stomach contents of seasonally co-occurring piscivores were examined to determine overlap in resource use for Summer Flounder (Paralichthys dentatus; 206–670 mm total length [TL]), Weakfish (Cynoscion regalis; 80–565 mm TL), Bluefish (Pomatomus saltatrix; 55–732 mm fork length [FL]), and Striped Bass (Morone saxatilis; 422– 920 mm FL). We collected samples from monthly, fishery-independent trawl surveys conducted on the inner continental shelf (5–27 m) off New Jersey from June to October 2005. Fish abundances and overlaps in diet and habitat varied over this study period. A wide range of fish and invertebrate prey was consumed by each species. Diet composition (determined from 1997 stomachs with identifiable contents) varied with ontogeny (size) and indicated limited overlap between most of the species size classes examined. Although many prey categories were shared by the piscivores examined, different temporal and spatial patterns in habitat use seemed to alleviate potential competition for prey. Nevertheless, the degree of overlap in both fish distributions and diets increased severalfold in the fall as species left estuaries and migrated across and along the study area. Therefore, the transitional period of fall migration, when fish densities are higher than at other times of the year, may be critical for unraveling resource overlap for these seasonally migrant predators.

Manuscript submitted 24 October 2012. Manuscript accepted 26 August 2013. Fish. Bull. 111:352–369. doi: 10.7755/FB.111.4.5 The views and opinions expressed or implied in this article are those of the author (or authors) and do not necesarily reflect the position of the National Marine Fisheries Service, NOAA.

Habitat and diet overlap of 4 piscivorous fishes: variation on the inner continental shelf off New Jersey Mark J. Wuenschel (contact author)1 Kenneth W. Able1 James M. Vasslides1 Donald M. Byrne2* Email address for contact author: [email protected] 1

Marine Field Station Rutgers University 800 c/o 132 Great Bay Boulevard Tuckerton, New Jersey 08087-2004 Present address for contact author: Northeast Fisheries Science Center National Marine Fisheries Service, NOAA 166 Water St. Woods Hole, Massachusetts 02543

2

New Jersey Department of Environmental Protection Nacote Creek Research Station P.O. Box 418 Port Republic, New Jersey 08241

*

Deceased

Predator species and their interactions with prey and other predator species play an important role in determination of the structure and function of ecosystems (Schmitz, 2007; Braga et al., 2012)—an especially important concern because populations of many predators have declined in abundance (Myers and Worm, 2003; Heithaus et al., 2008). In marine ecosystems, piscivorous fi shes have the potential to affect fish and invertebrate populations at lower trophic levels. In some cases, direct removals of prey resources by piscivorous fishes have been shown to rival or even exceed the removals by commercial fisheries (Buckel et al., 1999c; Overholtz et al., 2000; Overholtz and Link, 2007). Therefore, fish trophic ecology is relevant to several aspects of fisheries management (Link, 2002). With the general move toward multispecies and ecosystem-based approaches to fisheries management, there is a need for more comprehensive information on food web structure, interspecifi c

trophic interactions, and predator movements (Andrews and Harvey, 2013). In temperate zones, many coastal marine fishes undergo large-scale seasonal and ontogenetic shifts in their spatial distribution. Examples from the east coast of the United States include species that migrate north to New England in summer and south to the Carolinas in winter (e.g., Striped Bass [Morone saxatilis] and Bluefish [Pomatomus saltatrix]) and species that move inshore in summer and offshore in winter (e.g., Summer Flounder [Paralichthys dentatus] and Weakfish [Cynoscion regalis]) (Able and Fahay, 2010). Further, many temperate, estuarine-dependent species in the Middle Atlantic Bight leave estuaries in the fall of their first year to avoid cold winter temperatures (Able and Fahay, 1998; 2010). Many of these young-of-theyear (YOY) fishes are piscivorous and their egress from estuaries to coastal waters acts to concentrate them in time and space with other

Wuenschel et al.: Habitat and diet overlap of 4 piscivorous fishes

species or size classes, increasing the potential for both interspecifi c and intraspecifi c interactions. Interspecific competition and resource partitioning have been well documented for fish species in freshwater systems (Persson et al., 1999; Sutton and Ney, 2002; Bellgraph et al., 2008), where the potential for interactions may be greater than it is in marine systems given the closed nature of freshwater systems and fi sh populations. This interspecific competition and resource partitioning may apply to some degree in estuaries as well, as has been reviewed for European estuaries (Elliot and Hemingway, 2002). In contrast, because of the openness of marine populations and the ability of individuals to move great distances, interspecific competition in most marine systems likely is highly variable in time and space, making it more difficult to document and study interspecifi c competition in marine systems than in freshwater populations.

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Summer Flounder, Weakfish, Bluefish, and Striped Bass are important commercial and recreational species in New Jersey and elsewhere on the east coast of the United States. These species co-occur seasonally and feed on similar prey, indicating potential for competitive interactions. However, studies of food habits for these species generally have focused on estuarine collections (Gartland et al., 2006; Latour et al., 2008) or have been limited to seasonal, offshore (at depths of 5–400 m) collections aggregated over multiple years (Buckel et al., 1999b, Garrison and Link, 2000a, 2000b; Link et al., 2002; Overton et al., 2008; Woodland et al., 2011). Further, most prior studies on these species typically have focused on a single (Gartland et al., 2006; Latour et al., 2008) or a pair of species (Buckel and McKown, 2002; Buckel et al., 2009). Because of the spatial and temporal variability in competitive interactions between migratory fi shes, studies span-

A

B

Figure 1 (A) Study area where Summer Flounder (Paralichthys dentatus), Weakfish (Cynoscion regalis), Bluefish (Pomatomus saltatrix), and Striped Bass (Morone saxatilis) were sampled in 2005 along the northeastern coast of the United States for a 5-month study of habitat and diet overlap of these 4 piscivorous fishes and (B) sample collection area off New Jersey with the 15 strata outlined (strata were defined on the basis of latitudinal boundaries and depth contours of 9, 18, and 27 m). In June, August, and October, all strata were sampled. In July and September, only strata indicated with diagonal lines were sampled.

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ning multiple years, such as many of the ones listed above, potentially blur or miss finer-scale interactions that occur over shorter intervals. Therefore, to evaluate potential interactions between mobile, seasonally migratory species, information on spatial distributions and food habits is needed at finer spatial and temporal scales (Rudershausen et al., 2010). Although resource overlap among Summer Flounder, Weakfish, Bluefish, and Striped Bass has been indicated by or inferred in prior studies (Garrison and Link, 2000b), often over broad areas or time periods, a rigorous evaluation of these interactions at a more relevant ecological scale is lacking. The objectives for this study were to compare habitat use and food habits of 4 common predators of the Middle Atlantic Bight over the course of a 5-month period of co-occurrence in nearshore (inner continental shelf) waters. The degree of overlap in both habitat and diet between different size classes of these 4 predators was quantified to determine resource overlap at a fine spatial and temporal scale.

Materials and methods The distribution, abundance, and diet of Summer Flounder, Weakfish, Bluefish, and Striped Bass were evaluated from 20-min bottom trawls (30-m headrope, 6-mm codend liner [Byrne, 1994; Wuenschel et al.,

2012]) conducted in inner continental shelf waters of New Jersey in collaboration with the Bureau of Marine Fisheries of the New Jersey Department of Environmental Protection. Through the use of a depthstratified random sampling design, samples were taken during daylight hours at depths of 5 to 27 m along the New Jersey coast from the entrance of New York Harbor to the entrance of Delaware Bay (Fig. 1). The survey area was divided into 15 strata (Fig. 1) on the basis of latitudinal boundaries and depth contours (9, 18, and 27 m; Byrne, 1994). In June, August, and October 2005, all depths and strata were sampled with 2 tows per strata, plus 1 additional tow in each of the 9 largest strata (39 tows, Table 1; see Byrne 1994 for details). In the intervening months (July and September), sampling was undertaken with a bottom trawl net, bridles, and towing cables that were identical to the ones used in June, August, and October and were fished from the same vessel (RV Seawolf, SUNY Stonybrook) used during the other months, but because of constraints on vessel time, sampling was limited to 2 or 3 tows in nearshore and mid-shore depths for all but the northernmost and southernmost strata (18 tows, July; 12 tows, September; Table 1, Fig. 1). Representative subsamples, with a mean of 8.4 (9.6 standard deviation [SD]) per species per tow, of Summer Flounder, Weakfish, Bluefish, and Striped Bass were selected to cover the range of lengths of fish collected in a given tow for analysis of gut contents. Stomachs were re-

Table 1 Summary of samples collected in 2005 off the coast of New Jersey for this study of habitat and diet overlap of 4 piscivorous fishes. Collection month and sampling effort (numbers of tows in parentheses), numbers of fish collected, stomachs analyzed in the laboratory, and stomachs with prey in the gut for small, medium, and large Summer Flounder (Paralichthys dentatus), PdS, PdM, PdL; small, medium, and large Weakfish (Cynoscion regalis), CrS, CrM, CrL; small and large Bluefish (Pomatomus saltatrix), PsS, PsL; and Striped Bass (Morone saxatilis), Ms.

June (39)

July (18)

August (39)

September (12)

October (39)

All months (147)

Fish collected Stomachs analyzed Stomachs with prey Fish collected Stomachs analyzed Stomachs with prey Fish collected Stomachs analyzed Stomachs with prey Fish collected Stomachs analyzed Stomachs with prey Fish collected Stomachs analyzed Stomachs with prey Fish collected Stomachs analyzed Stomachs with prey

PdS

PdM

PdL

85 47 30 134 72 39 302 71 64 29 4 3 12 6 3 562 200 139

170 63 42 296 153 74 950 245 178 783 117 74 192 134 79 2391 712 447

80 35 26 61 51 12 148 108 62 116 22 8 31 26 13 436 242 121

CrS 1 0 0 64 0 0 2587 100 99 17,836 134 96 9746 172 130 30,234 406 325

CrM

CrL

PsS

PsL

Ms

774 46 42 6319 125 116 4232 109 84 11,752 58 45 10,275 93 75 33,352 431 362

1 1 1 0 0 0 31 7 5 189 5 5 1786 86 66 2007 99 77

9 0 0 48 21 18 890 150 105 2370 185 147 616 156 130 3927 512 400

4 6 4 3 3 3 23 20 17 6 5 2 70 67 52 112 101 78

7 0 0 0 0 0 214 27 11 0 0 0 123 89 37 344 116 48

Wuenschel et al.: Habitat and diet overlap of 4 piscivorous fishes

moved immediately after capture and preserved in formalin for laboratory analysis. In some months, tagging and releasing Striped Bass was a higher priority than determining stomach contents; therefore, all fishes captured were not available for diet analysis. To account for size-related changes in habitat use (Able and Fahay, 2010), diet composition within species (Garrison and Link, 2000b), and interactions across species (Buckel and McKown, 2002), species were split into multiple size classes when data permitted: small (Summer Flounder: 200–300 mm total length [TL]; Weakfish: 80–200 mm TL; Bluefish: 55–300 mm fork length [FL]), medium (Summer Flounder: 301–400 mm TL; Weakfish: 201–350 mm TL), and large (Summer Flounder: 401–670 mm TL; Weakfish: 351–565 mm TL; Bluefish: 301–732 mm FL). For Striped Bass, a single size class was used because of limited sample sizes, the absence of prior evidence for ontogenetic shifts beyond the YOY stage (Walter et al., 2003), and the relatively large sizes of our specimens (422–920 mm FL). Diet analysis In the laboratory, preserved stomachs were carefully opened and the contents transferred to a solution of rose bengal stain and 95% ethyl alcohol. Prey items were identified to the lowest practical taxonomic level by using available keys and guides for the Mid-Atlantic region (Weiss, 1995; Able and Fahay, 1998) and enumerated. For each stomach, abundant or large prey types were sorted and placed on preweighed filter papers or aluminum weighing pans and dried to a constant weight (+0.0001 g) in a drying oven (70ºC). Dry weights were chosen because they are more representative of nutritional value and have less weighing error than wet weights (Hyslop, 1980), especially for small or partial prey (Carr and Adams, 1972). For small and, therefore, hard-to-separate prey items (e.g., copepods and mysids), an aggregate sample was dried and the percent contribution by volume of different prey types was recorded and later converted to weights. Through the use of this protocol, prey-specific dry weights were obtained directly for larger prey or estimated from aggregate samples of smaller, mixed prey items for each stomach analyzed. Trawl collections yielded “clusters” of individuals within species and size classes per location; therefore, the percent contribution by weight of prey items was calculated with the following cluster sampling estimator (Buckel et al., 1999a; 1999b; Gartland et al., 2006). For a given size class of a predator, the percent contribution by weight of each prey type k (%Wk) to the diet was calculated with the following equation: n

%Wk =

∑ Mi qik

i=1 n

∑ Mi

i=1

• , •100

(1)

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Wik where qik = ; Wi n = the number of trawls; Mi = the number of species size class sampled per tow i; wi = the total dry weight of all prey in stomachs for that species size class in tow I; and wik = the total dry weight of prey type k in all stomachs for that species size class collected in tow i. To facilitate analysis, prey items were grouped into the following general categories: squids (predominantly Loligo spp.), decapod crustaceans (including swimming crabs, sand crabs, rock crabs [Cancer borealis and C. irroratus], spider crabs, hermit crabs, decapod zoea, and shrimps [predominately Crangon septemspinosa and Palaemonetes sp.]), nondecapod crustaceans (including amphipods, isopods, cumaceans, mysids, and mantis shrimp), bivalves (clams and periwinkles), fishes (44 species identifi ed), worms and wormlike organisms (nematodes, polychaetes, annelids, and leeches), and other unidentified (UID) items (inorganic matter, organic matter, eggs, and insects). In addition, prey items (species or higher taxa) that contributed on average >5% by weight to the overall mean diet of a species size class were included as additional prey categories. Therefore, if Bay Anchovy (Anchoa mitchilli) composed >5% of the diet for a given species size class in any month, it was included as a prey category and the category “fishes” represented all remaining fish prey that contributed 40 sampled guts were included in the similarity analysis (described in the next paragraph). To evaluate the degree of similarity in diets between species and size classes, nonmetric multidimensional scaling (nMDS) and hierarchical clustering were used

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within each month and across all months (PRIMER-E, Ltd., Plymouth, UK1). The nMDS data were calculated as the percentage of diet by weight for each month and each species-size-class combination and were log-transformed before use in the Bray-Curtis index to construct the sample similarity matrix. Group-average hierarchical clustering was then used to identify those predators that had dietary similarities at the 60% level following Jaworski and Ragnarsson (2006) and Clarke and Warwick (2001). Habitat and diet overlap Habitat and diet overlap between pairs of species size classes were determined through the use of Schoener’s index (Schoener 1970). This index was calculated with this equation: n

α = 1 − 0.5 ( ∑ pij − pik ),

(3)

i=1

which shows the overlap (α), where pij and pik are the proportions of the ith resource (trawl station or prey proportion) used by species j and k, respectively. Index values range from 0 to 1, with values >0.6 representing biologically important overlap in resource use (Wallace, 1981; Buckel and McKown, 2002; Bethea et al., 2004).

Results Spatial distribution, abundance, and sizes The spatial distribution, abundance, and size distribution for each of the 4 predators were variable over the course of our 5-month study (Table 1, Figs. 2 and 3). Summer Flounder were the most consistently collected species throughout this study. They were distributed throughout our study area, with a slight shift in abundance from offshore to inshore in summer followed by the reverse in fall. The size distribution of Summer Flounder was relatively constant, with individuals of 206–670 mm TL representing a broad range of age classes (YOY to 4+) collected from June to October. Weakfish (80–565 mm TL) also were collected consistently, with greater abundances occurring inshore. Catches of Weakfish in June and July were dominated by larger size classes (>200 mm TL), with smaller size classes (YOY or 1+, ≤200 mm TL) becoming abundant from August to October. Similarly, Bluefish (55–732 mm FL) were most abundant inshore. They were dominated by larger size classes (>300 mm FL) in June, with smaller size classes (≤300 mm FL) becoming abundant inshore from July to October. Striped Bass, which were typically larger (422–920 mm FL) than the other 3 species (Fig. 3), were less abundant and highly variable in time and space. In 1

Mention of trade names or commercial companies is for identification purposes only and does not imply endorsement by the National Marine Fisheries Service, NOAA.

June and August, collections of Striped Bass were limited to the northernmost strata of the sample area. No Striped Bass were collected in July and September, likely because the northern strata were not sampled during those months. In October, Striped Bass were distributed throughout our study area. In all months in which they were collected, they were typically inshore. Diet analysis Because of limited or zero abundance of some species size classes in some months (described previously), sampling limitations, and the occurrence of empty stomachs, adequate food habit information was not obtained over all species size classes or months. The relative contributions of prey types to the diets for species size classes in each month are summarized in Table 2 (species size classes with insufficient samples sizes to be included in subsequent analyses are presented). For the groups that were considered adequately sampled for diet description (Fig. 4) and, therefore, included in the cluster analysis, the cumulative trophic diversity curves indicated that sample sizes of 30–40 guts were sufficient to characterize the diet in most cases. However, for some monthly species size classes (e.g., medium Weakfish in August and medium Summer Flounder in October) trophic diversity continued to increase beyond 40 guts analyzed. Size-specific patterns in trophic diversity differed across species, with Summer Flounder showing decreased diversity with size in June. In contrast, larger size classes of Weakfish in August and Bluefish in October had more diverse diets than did smaller size classes of these species. Overall, diversity of prey items increased throughout time in Summer Flounder and Striped Bass, and it remained low for small Bluefish. Weakfish diet diversity also was relatively stable through the 5-month period of this study, with the exception of the high trophic diversity of the medium size class of this species in August. Cluster analysis separated the size classes for each of the 4 species into 3 groups in June at the 60% similarity level (Fig. 5). The first group consisted of large (>400 mm TL) Summer Flounder, which preyed predominantly on squids (88.1%) (Table 2). Small and medium Summer Flounder formed the second group, and medium (201–350 mm TL) Weakfish the third. Although the second and third groups consumed mostly fishes (0–73.3% sand lances [Ammodytes spp.] and 6.1– 52.9% UID and other fishes), they were separated by the amounts of decapod crustaceans (2.1–20.9%) and squids (8.3–21.5%) in the former and mysids (33.4%) in the latter. In July, the cluster analysis identified 3 groups at the 60% similarity level from among the 4 species size classes for which enough data were available. Small and medium Summer Flounder were grouped together, with diets consisting of both pelagic and benthic prey, including Butterfish (Peprilus triacanthus; 15.8–18.7%),

Wuenschel et al.: Habitat and diet overlap of 4 piscivorous fishes

June

July

357

August

September

October

Figure 2 Distributions of Summer Flounder (Paralichthys dentatus), Weakfish (Cynoscion regalis), Bluefish (Pomatomus saltatrix), and Striped Bass (Morone saxatilis) sampled in June, July, August, September, and October of 2005 along the coast of New Jersey for this study. Circles are shaded by species and represent abundance (log transformed), with the same scale in all frames, and crosses indicate zero catches. See Table 1 for a summary of numbers caught for each species.

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Weakfish

Bluefish

Striped Bass

Frequency (%)

Summer Flounder

Total length (mm)

Total length (mm)

Fork length (mm)

Fork length (mm)

Figure 3 Length distributions (percent frequency) for Summer Flounder (Paralichthys dentatus), Weakfish (Cynoscion regalis), Bluefish (Pomatomus saltatrix), and Striped Bass (Morone saxatilis) collected from June (top row) to October (bottom row) in 2005 off the coast of New Jersey for this study. Bars are shaded by species (as in Fig. 2). Vertical, dashed gray lines indicate breakpoints between size classes. Note that the x-axis scales are different across species and the y-axis scales are different across months within species.

Wuenschel et al.: Habitat and diet overlap of 4 piscivorous fishes

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Table 2

Jun

Jul

Aug

Sep

Oct

All months

CrM CrL* PdS PdM PdL PsL* CrM PdS PdM PdL* PsS PsL* CrS CrM CrL* Ms PdS PdM PdL PsS PsL* CrS CrM CrL* PdS* PdM PdL* PsS PsL* CrS CrM CrL Ms PdS* PdM PdL* PsS PsL CrS CrM CrL Ms PdS PdM PdL PsS PsL

42 52.9 1 30 10.0 21.8 2.7 42 1.5 73.3 0.8 6.1 4.9 26 0.3 10.3 0.1 4 94.0 116 15.6 1.5 0.9 4.2 0.1 1.7 1.6 39 4.2 16.4 18.7 24.7 2.7 1.8 2.6 74 2.9 0.9 9.8 15.8 35.3 23.8 0.3 3.0 12 1.4 61.6 24.3 9.5 18 29.9 31.3 38.8 3 19.1 52.6 28.2 99 5.1 4.8 42.1 0.2 84 7.6 10.0 23.5 28.5 1.2 1.6 5 10.9 54.2 10.1 11 2.2 6.1